The present invention relates to an apparatus and a method suitable for performing carburization of steel parts for use, for example, in automotive industry and/or machinery industry.
In order to rapidly raise the temperature of a treatment object to a carburizing temperature when the treatment object is induction heated in a carburizing apparatus, a power source having high rated capacity has to be connected to the induction heating coil. In particular, the power source capacity has to be increased to raise the carburizing temperature in order to shorten the carburizing time. However, power sources having high rated capacity are large and expensive; therefore if the number of coils is increased to increase the throughput, the number of power source and the installation space are increased and the manufacturing and/or installation cost of carburizing apparatus increases.
Accordingly, it is suggested to use a carburizing apparatus in which a treatment object is heated to a carburizing temperature in a preheating chamber, treatment objects are maintained at a carburizing temperature by induction heating in a plurality of carburizing chambers, then carburization is carried out (see Patent Document 1). The carburizing time thus can be shortened by continuously treating a plurality of treatment objects.    Patent Document 1: Japanese Patent Application Laid-open No. 10-53809
However, in the conventional carburizing apparatus, doors are provided so as to be opened and closed between the preheating chamber and each carburizing chamber, the doors are closed after the treatment object heated to the carburizing temperature in the preheating chamber is conveyed into the carburizing chamber, then gas for carburizing atmosphere is caused to flow at a set flow rate into the carburizing chamber, and then the electric current is passed through the coil in the carburizing chamber, thereby heating the treatment object. As a result, the interval from the completion of heating of the treatment object in the preheating chamber to the start of carburization in the carburizing chamber is increased, so that the drop in temperature of the treatment object during this interval is increased. The resultant problem is that a long interval is required to heat the treatment object again to the carburizing temperature in the carburizing chamber and the carburizing time cannot be sufficiently shortened.
Furthermore, in a case where the treatment object has magnetic properties when the induction heating is started in the carburizing chamber, if the temperature of the treatment object becomes less than the Curie point, a mechanism is necessary to prevent the treatment object from jumping up under the effect of electro-magnetic force when the electric current is passed through the coil in the carburizing chamber. A problem resulting from providing such jumping preventing mechanism in each of a plurality of carburizing chambers is that the structure of the apparatus becomes complex and the manufacturing cost increases.
It is an object of the present invention to provide a carburizing apparatus and a carburizing method that can resolve the above-described problem.